/Engine/Source/Runtime/UMG/Public/Blueprint/WidgetBlueprintGeneratedClass.h
This is the function that makes UMG work. Once a user widget is constructed, it will post load call into its generated class and ask to be initialized. The class will perform all the delegate binding and wiring necessary to have the user's widget perform as desired.
void InitializeWidget ( [UUserWidget](API\Runtime\UMG\Blueprint\UUserWidget) * UserWidget ) const